1. Field of the Invention
This invention relates generally to an anode purge valve for a fuel cell stack and, more particularly, to a single anode purge valve for a split fuel cell stack, where the purge valve is positioned between the split stacks.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of flow field or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of the MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. The bipolar plates also include flow channels through which a cooling fluid flows.
For automotive applications, it typically takes about 400 fuel cells to provide the desired power. Because so many fuel cells are required for the stack in automotive fuel cell system designs, the stack is sometimes split into two sub-stacks each including about 200 fuel cells because it is difficult to effectively provide an equal flow of hydrogen gas through 400 fuel cells in parallel.
Also, the membrane within a fuel cell needs to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. This humidification may come from the stack water by-product or external humidification. The flow of hydrogen through the anode gas flow channels has a drying effect on the membrane, most noticeably at an inlet of the hydrogen flow. Also, the accumulation of water droplets within the anode gas flow channels from the membrane relative humidity and water by-product could prevent hydrogen from flowing therethrough, and cause the cell to fail because of low reactant gas flow, thus affecting the stack stability. The accumulation of water in the reactant gas flow channels is particularly troublesome at low stack output loads.
It may be desirable in certain fuel cell system designs to purge the anode side of the fuel cell stack at system shut-down with air from the compressor to remove the remaining hydrogen and water in the flow channels in the anode side of the stack. Removing the hydrogen from the anode side at system shut-down has certain well-known benefits, and removing the water from the flow channels prevents it from freezing in the stack in low temperature environments.
Traditionally, for a split fuel cell stack design, there is a separate anode purge valve for both split stacks positioned between the stacks to provide the compressor air to the anode channels at system shut-down. It has been proposed in the art to eliminate one of the valves and provide a single valve having a single output directed to both split stacks. However, with a single valve purge system, there is a flow link between the anode side of the split stacks through the single purge valve that affects system performance during normal fuel cell stack operation. Therefore, it would be beneficial to provide a single purge valve for a split stack design that was appropriately sized for the size the system, and eliminated the flow path through the valve to the anode sides of the split stacks.